1. Field of the Invention
The present invention generally relates to a sensing circuit, and more particularly to a sensing circuit for a capacitive touch panel.
2. Description of Prior Art
Applications of conventional touch panels are widely used, for example, mobile phones, touch screens for public information, automatic teller machines (ATMs) and so on. The intuitive operations of a touch panel can be substituted for the functions of a keyboard and a mouse; therefore, the touch panel is quite convenient in use.
Please refer to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 illustrate two types of capacitive touch panels 150, 250 for detecting a touch position. In FIG. 1, the capacitive touch panel 150 comprises a plurality of sensing lines in the X-axis direction and in the Y-axis direction, that is, the sensing lines X1-X4 and Y1-Y4. Each of the sensing lines X1-X4 and Y1-Y4 has a plurality of sensing electrodes (not shown). The sensing capacitance of each sensing electrode is represented as CSENSE. When no touch event occurs, the sensing capacitance CSENSE is zero. When a touch event occurs, the sensing capacitance CSENSE is not zero. The capacitive touch panel 150 in FIG. 1 detects the touch position by sequentially scanning each of the sensing lines X1-X4 and Y1-Y4. As shown in FIG. 1, the scanning sequence is from the sensing line X1 to the sensing line X4, and then from the sensing line Y1 to the sensing line Y4. When the intersection point of the sensing line X4 and the sensing line Y1 is touched, for example, the touch position can be detected by a change of the sensing capacitance CSENSE.
In the other sensing method, sensing lines in only one direction are scanned, and a stimulating signal is inputted to sensing lines in the other direction. As shown in FIG. 2, the capacitive touch panel 250 also comprises a plurality of sensing lines in the X-axis direction and in the Y-axis direction, that is, the sensing lines X1-X4 and Y1-Y4. The scanning sequence is from the sensing line X1 to the sensing line X4, and the stimulating signal is sequentially inputted from the sensing line Y1 to the sensing line Y4. That is, the stimulating signal is inputted to the sensing line Y1, and the scanning sequence is from the sensing line X1 to X4. Then, the stimulating signal is inputted to the sensing line Y2, and the scanning sequence is from the sensing line X1 to X4. The rest processes can be done in the same manner. A sensing capacitance CTRANS represents a coupling capacitance between the sensing line X1 and the sensing line Y1. The sensing capacitance CTRANS has different values at the intersection point of the sensing line X1 and the sensing line Y1 being touched or untouched. Similar to FIG. 1, the touch position can be detected by a change of the sensing capacitance CTRANS.
Please refer to FIG. 3. FIG. 3 illustrates a block diagram of a conventional sensing circuit 100. The sensing circuit 100 comprises a sensing unit 102 (a sensing electrode), a sensing signal generating unit 104, and an integrator 106. The sensing unit 102 has the sensing capacitance CSENSE as shown in FIG. 1 or the sensing capacitance CTRANS as shown in FIG. 2 to indicate that a touch event has occurred or not. When the capacitive touch panel 150 in FIG. 1 or the capacitive touch panel 250 in FIG. 2 is touched, the sensing capacitance of the sensing unit 102 is changed. The sensing signal generating unit 104 generates a sensing signal according to the change of the sensing capacitance. The sensing signal is a voltage signal and inputted to the integrator 106 to be integrated. Finally, the integrator 106 outputs an integrated result, and the touch position can be determined by the integrated result.
However, regardless if it is the capacitive touch panel 150 in FIG. 1 or the capacitive touch panel 250 in FIG. 2, there exists a problem that a noise affects the sensing signal. As it is known from the prior arts, the sensing unit 102 is disposed on a sensing electrode substrate (not shown), and the sensing signal generating unit 104 and the integrator 106 are disposed on an array substrate of a liquid crystal display panel (not shown). A common electrode (not shown) is disposed between the sensing electrode substrate and the array substrate of the liquid crystal display panel for providing a required voltage level when the liquid crystal display panel operates. When the liquid crystal display panel operates, a common voltage provided by the common electrode is not clear. That is, the common voltage has the noise. For example, the noise is generated when a source bus of the liquid crystal display panel is driven to be pre-charged. In addition, the noise is also generated when switches of the liquid crystal display panel are turned on and off. The noise enters the sensing circuit 100 in FIG. 3, causing the sensing circuit 100 to fail to detect the touch position or resulting in detection errors.
Therefore, there is a need to solve the above-mentioned problem that the noise affects the sensing signal.